Activists demonstrating during Canadian prime minister Stephen Harper's visit to Parliament have been arrested.

Scotland Yard said three people were arrested after a protest at the Sovereign's Entrance.

Environmental campaigners against "tar sands", a form of fuel which produces more emissions than conventional oil, have posted a video showing them on the roof of Parliament, with a statement saying they wanted to disrupt the premier's address to MPs and peers.

We have entered Parliament to interrupt Harper's speech. We have managed to climb onto the roof with t-shirts saying 'oil out of politics', 'stop Harper' and 'stop the tar sands'. Two campaigners have spilled molasses on the floor outside of Parliament.

– The "Love Canada Hate Tar Sands" website.

Shortly before Mr Harper's speech the doors slammed shut on the robing room, where the Queen puts on the Imperial State Crown and her ceremonial robes for the state opening of Parliament, and shouting was heard.

The Crown Prosecution Service has said there was "insufficient evidence" that Eric Joyce MP had committed an offence after he was arrested on suspicion of assault and ABH following an incident outside a bar in the House of Commons.

Although there appears to have been an incident on the evening in question, evidence obtained from witnesses contained multiple inconsistencies and there is insufficient evidence that Mr Joyce committed any offence.

On this basis, we have concluded there is insufficient evidence to support a realistic prospect of conviction and that no further action should be taken.